Have you ever noticed that a beach seems to "shrink" over the course of a day? In the morning the strip of sand is wide; in the afternoon the water reaches almost to the tent. Nobody pushed the sea. What happened was the tide: the entire ocean rose and fell, obeying a force that comes from way up above.
That force is the Moon's gravity — with a smaller assist from the Sun. It's the same pull that keeps the Moon orbiting the Earth, only acting on the water. In this article you'll understand why the sea rises, why there are two high tides a day, why the full moon stirs the sea more, and how to read the tide table so your Club isn't caught off guard on a rocky shore.
Watch out for an important distinction: here the subject is the Moon pulling the water by gravity. Don't confuse it with the phases of the Moon (which are about light and shadow) or with the ocean's marine life. Today's theme is mechanical: mass, distance, and pull.
Why does the sea rise and fall?
Everything with mass attracts everything with mass. The Earth attracts you (that's why you don't float away). The Moon, even nearly 384 thousand kilometers away, also attracts the Earth — and pulls the ocean water with a little more force on the side facing it, because that side is closer.
That extra little pull gathers water into a wide, low mound, a kind of ocean "bulge" pointed at the Moon. That's high tide. It's good to keep the scale in mind: it's not a giant wave traveling across the sea. It's the average level of the entire ocean that rises a few centimeters or a few meters, slowly, over hours.
The Sun does the same thing. It's gigantic, but it's very much farther away, so its effect on the tides is much smaller — about 46% of the Moon's effect, according to NOAA (the U.S. ocean agency). In other words: the Moon sets the rhythm, and the Sun plays a supporting role. Keep this in mind, because it'll come back into the story shortly.
Why are there TWO high tides a day?
This is the question that confuses almost everyone. If the Moon pulls the water toward its side, there should be just one bulge, on the Moon's side. But there are two, on opposite sides of the Earth. Why?
The secret is the difference in pull. The Moon pulls the side of the Earth closest to it more strongly, pulls the center of the Earth moderately, and pulls the farthest side more weakly. The result: on the near side the water is pulled toward the Moon; on the far side, the water is left "behind" (it's pulled less than the rest of the Earth) and piles up there too. There are two bulges, one on each side of the planet.
Now add the Earth's rotation. The planet spins beneath these two bulges. Each point on the coast passes through a bulge, then through a low region, then through the other bulge, then through another low region — with each rotation. That's why most coastlines have two high tides and two low tides a day, with about 6 hours between a high tide and the following low tide.
A fine detail: it's not every exactly 24 hours. While the Earth spins, the Moon also moves a little in its orbit. The Earth has to spin a tiny bit more to "catch up" with the Moon. That's why the cycle is 24 hours and 50 minutes, and the high tides fall about 12h25 apart. That's why the tide "runs late" by roughly 50 minutes every day — today the high tide was at 9:00, tomorrow it will be near 9:50.
Spring and neap tides: when the sea pulls harder
Here the Sun comes back into play. On its own it's weak, but when it lines up with the Moon, the two pulls add together. This happens at the new moon and the full moon: Sun, Earth, and Moon are almost in a straight line. The two bulges get higher and the lows get lower. That's the spring tide — the strongest one, with the greatest difference between rising and falling.
When the Moon is in the first or last quarter, it forms a 90° angle with the Sun as seen from Earth. Then one pull works against the other. The tides get weaker, with little difference between high and low. That's the neap tide, or slack-water tide.
Notice that this ties directly to the phases of the Moon, but for a different reason than the light. The phase shows the Sun–Earth–Moon alignment in the sky; the spring tide is the gravitational consequence of that same alignment in the sea. A beautiful full moon in the sky usually comes together with a strong high tide at the beach. If your Club enjoys observing the sky, this is a natural bridge between astronomy and the ocean — it's worth pairing with a night of observing the night sky.
Practical rule for camping: on full-moon or new-moon days, expect the water to rise more than usual. That's precisely when poorly placed tents on the sand wake up wet.
Read alsoThe phases of the moon explainedSafety: how not to be caught off guard by the tide
This is the part that can save a life. The tide rises slowly and silently, and that's exactly why it's treacherous. Nobody "sees" the tide coming the way they see a wave. You look at the dry rocks, cross over, explore the pools — and when you come back, the dry path has become a deep channel with a current. Rocky shores, sandbars, and beach caves are the places where this happens most.
The tool is the tide table. It's a table — published in Brazil by the Navy, through the Directorate of Hydrography and Navigation — that gives the time and height of each high and low tide, day by day, for each port along the coast. Before any activity on a beach or rocky shore, the Counselor or the Director consults the table for that place and that date. Without it, don't head down to the rocks.
Set up the Club's rule like this: find out the time of the next high tide; plan to be back on solid ground with a margin of at least one hour before it; never leave a Pathfinder alone at a spot the water can cut off; and agree on a high, dry meeting point. If someone gets stranded, the guidance is not to try to swim across a tidal current — asking for help and waiting in a safe, visible place is safer.
Add to that the basics of water safety: life vests when appropriate, frequent head counts, and never turning your back on the sea on a rocky shore. A drowning emergency or a fall is a call to your local emergency number (in Brazil, SAMU 192, or Fire Department 193). This text is awareness — it does not replace a water safety course or the judgment of a trained adult on site.
Trivia: mangroves, fishing, and life in the tide's rhythm
The tide isn't just a camping problem — it's the clock of entire ecosystems. The mangrove, for example, is a strip that is submerged at high tide and exposed at low tide. Crabs, swimming crabs, and birds learned to live in that back-and-forth: at low tide the mud becomes a bountiful table of food; at high tide the fish come in to feed and hide among the roots.
Anyone who fishes has known this in practice for generations. Many fish move and feed more during the turning of the tide, when the water is running. Small-scale fishermen plan their trips by the tide table as much as by the weather forecast. It's traditional knowledge that lines up perfectly with the science of gravity.
There's also the tide pool, that miniature world that forms in the hollows of the rocky shore when the water drops: starfish, anemones, small fish, and snails trapped until the tide returns. It's a perfect living laboratory for an observation activity — as long as, you already know, with one eye on the tide table and the other on the clock.
Who set a limit for the sea
For the Pathfinder, understanding the mechanism doesn't erase the wonder — it increases it. The Bible uses exactly the image of the sea that rises, but goes no further than a limit. In the book of Job, God asks: "Or who shut up the sea with doors... when I fixed my limit for it and set bars and doors, and said, 'This far you may come, but no farther, and here your proud waves must stop'?" (Job 38:8-11, NIV).
Jeremiah repeats the idea with an image that matches what you saw in this article — the sand of the beach as a boundary: "I made the sand a boundary for the sea, an everlasting barrier it cannot cross. The waves may roll, but they cannot prevail" (Jeremiah 5:22, NIV).
The Adventist reading of these texts is simple: the same God who created gravity, the Moon, and the orbit is the one who sustains the order that makes the tide rise and stop every day, without fail. Science describes the "how"; faith answers the "who." This is an offer of meaning, presented with respect — you are free to think about it. But perhaps, the next time the water reaches the line of wet sand and pulls back, you'll remember that it has had a set time for a very long while.